Among animals, there exists a single known example of an apparently freely rotating structure, though it is used for digestion rather than propulsion: the crystalline style of certain bivalves and gastropods.[19]: 89 The style consists of a transparent glycoprotein rod which is continuously formed in a cilia-lined sac and extends into the stomach. The cilia rotate the rod, so that it becomes wrapped in strands of mucus. As the rod slowly dissolves in the stomach, it releases digestive enzymes.[19] Estimates of the speed of rotation of the style in vivo vary significantly, and it is unclear if the style is rotated continuously or intermittently.[20]
Weird, kinda gross, and fascinating!
